Abstract. In situ high resolution aircraft measurements of cloud microphysical
properties were made in coordination with ground based remote sensing
observations of a line of small cumulus clouds, using Radar and Lidar, as
part of the Aerosol Properties, PRocesses And InfluenceS on the Earth's
climate (APPRAISE) project. A narrow but extensive line (~100 km long)
of shallow convective clouds over the southern UK was studied. Cloud top
temperatures were observed to be higher than −8 °C, but the clouds
were seen to consist of supercooled droplets and varying concentrations of
ice particles. No ice particles were observed to be falling into the cloud
tops from above. Current parameterisations of ice nuclei (IN) numbers predict
too few particles will be active as ice nuclei to account for ice particle
concentrations at the observed, near cloud top, temperatures
(−7.5 °C).

The role of mineral dust particles, consistent with concentrations observed
near the surface, acting as high temperature IN is considered important in
this case. It was found that very high concentrations of ice particles (up to
100 L−1) could be produced by secondary ice particle production
providing the observed small amount of primary ice (about 0.01 L−1) was
present to initiate it. This emphasises the need to understand primary ice
formation in slightly supercooled clouds. It is shown using simple
calculations that the Hallett-Mossop process (HM) is the likely source of the
secondary ice.

Model simulations of the case study were performed with the Aerosol Cloud and
Precipitation Interactions Model (ACPIM). These parcel model investigations
confirmed the HM process to be a very important mechanism for producing the
observed high ice concentrations. A key step in generating the high
concentrations was the process of collision and coalescence of rain drops,
which once formed fell rapidly through the cloud, collecting ice particles
which caused them to freeze and form instant large riming particles. The
broadening of the droplet size-distribution by collision-coalescence was,
therefore, a vital step in this process as this was required to generate the
large number of ice crystals observed in the time available.

Simulations were also performed with the WRF (Weather, Research and
Forecasting) model. The results showed that while HM does act to increase the
mass and number concentration of ice particles in these model simulations it
was not found to be critical for the formation of precipitation. However, the
WRF simulations produced a cloud top that was too cold and this, combined
with the assumption of continual replenishing of ice nuclei removed by ice
crystal formation, resulted in too many ice crystals forming by primary
nucleation compared to the observations and parcel modelling.